High performance antennas are becoming increasingly prevalent as spacecraft, aircraft, ship, and ground vehicle mission requirements become more sophisticated. In particular, greater precision in simulation and design of antennas, transmitters and end items antennas and transmitters are mounted upon is needed for a variety of reasons.
One problem in the development and manufacture of antennas is the accurate measurement of antenna performance. Traditionally, antenna performance measurement was conducted by placing the antenna at a remote location and measuring the amplitude response characteristics as a function of orientation of the antenna throughout its operational range. Required measurement distances for high gain antennas range from fifty feet to three miles or more. This measurement technique, known as far-field testing, suffers from significant practical limitations, such as susceptibility to the effects of weather, ground reflections, and increasing real estate costs.
Near-field testing was developed as an alternative to far-field testing. A typical near-field measurement system consists of three primary subsystems: a computer, a robotic positioner, and a probe. The computer provides the user interface and controls the operation of the probe. In addition, it commands the robotic positioner which moves the antenna under test (AUT), the probe, or both, over the desired virtual surface. The probe can be used to transmit or receive a radio frequency (RF) signal to/from the AUT. Consequently, for reciprocal antennas in near-field testing, the probe can transmit or receive and the AUT performs the corresponding opposite function (receiving or transmitting). One skilled in the art will appreciate that whenever an antenna is a passive linear reciprocal device, the use of the antenna in either a transmitting situation or a receiving situation is implicit. However, when an antenna or group of antennas is combined with circuit elements that are active, nonlinear, or nonreciprocal, the pattern will likely be different between transmit and receive states and so both conditions should be tested. Examples of such cases are an adaptive antenna system and a signal-processing antenna system, conical-scanning, monopulse, or compound interferometer systems. The near-field testing equipment records the amplitude and phase of a RF signal received for a series of discrete data points across a virtual surface of an AUT. This data is transformed into a far-field pattern associated with an AUT.
The two types of testing have trade-offs. With far-field testing only amplitude measurements are required (not phase) in order to derive far-field antenna patterns. Consequently, far-field testing is generally a better choice for lower frequency antennas and simple pattern cut measurements. In contrast, both amplitude and phase measurements are required for near-field testing, making it a better choice for higher frequency antennas and complete pattern and polarization measurements. Moreover, there are significant logistical limitations.
Near-field testing typically has been conducted in an indoor test facility utilizing fixed, rigid equipment with an AUT mounted on a stable, rigid fixture. In near field testing, knowledge of the precise locations of the probe and an AUT when a discrete amplitude and phase data points are recorded is a critical element. Relative movement (e.g. vibration) between the probe and an AUT may introduce inaccuracies/errors into the process. Typically, if displacement due to relative vibration (or other position errors) exceeds 1/100 of the RF wavelength, accuracy of the near-field test will be reduced. Therefore, near-field test systems typically employ precise positioning components in order to minimize error. This form of testing is cumbersome and expensive. First of all it requires the presence of the AUT at the test facility. Depending on the location of the antenna to be tested, the cost of its transportation to and from the test facility, and the opportunity cost while the antenna is out of service, the indoor testing process is not always a cost-effective means of evaluating an antenna. Far field testing methods also require knowledge of the location of the probe and the AUT when a discrete amplitude data point is recorded, but the need for accuracy is not as critical as with near-field testing. On the other hand, far-field testing requires large far-field test ranges, where reflections from buildings, vegetation, seasonal changes, rain, snow and ice have an effect on measurement results and repeatability of the results. For vehicle-mounted antennas, the vehicles are usually mounted on large-diameter turntables that orient the vehicle and an AUT during testing. The vehicle is not in its normal environment (over real ground), and the turntable itself can introduce error. Moreover, these large test ranges and turntables are expensive infrastructure.
Various positioning systems can be used in combination with an antenna testing system. For example, Global Positioning System (GPS) technology provides positioning information. However, traditional GPS systems used in connection with antenna test systems per se do not provide sufficient information to provide adequate testing results.
Another example of positioning systems includes Real Time Kinematics (RTK) systems, which are a particular kind of GPS in which the positional data are displayed and recorded. RTK/GPS systems can be used to improve antenna test systems in order to provide the requisite level of test precision under a wide variety of real world conditions.